Depression:
Depression is a mood disorder characterized by depressed mood; feelings of worthlessness, helplessness or hopelessness; a loss of interest or pleasure; changes in appetite; change in sleeping pattern; fatigue, thoughts of death, inability to concentrate or make decisions. According to Statistics Canada's 2002 Mental Health and Well-being Survey4, 12.2% of all Canadians will experience depression within their lifetime, while 4.8% of Canadians had reported symptoms for major depression. Furthermore, the economical impact of depression is tremendous due to costs in both productivity and health care. In Canada, between 62% and 76% of short-term disability episodes due to mental disorders were attributed to depression5. Work-related productivity losses due to depression have been estimated to be $4.5 billion6. Thus, the prevalence of depression makes this disorder a very important health issue in Canada and abroad.
Once diagnosed, depression can be treated by different therapies including medication, psychotherapy and in more severe cases, with electroconvulsive therapy. The first line of treatment is often through antidepressant medication, sometimes in conjunction with psychotherapy. Antidepressants consist of the classical tricyclic antidepressants (TCA), selective serotonin reuptake inhibitors (SSRI), noradrenaline and serotonin reuptake inhibitor (NSRI), as well as monoamine oxidase inhibitors (MAOI). All antidepressants have acute effects on synaptic levels of neurotransmitters in the brain7-9. The classical TCAs are predominantly noradrenaline and serotonin reuptake inhibitors, similar to NSRIs. The SSRI drugs are more selective serotonin transporter inhibitors, while MAOI block enzymes that are involved in the breakdown of these neurotransmitters.
Dopamine (DA), acting through DA receptors, exerts a major role in regulating neuronal motor control, cognition, event prediction, emotion and pleasure/reward10-15, all of which are affected in depression. The contribution of DA in depression becomes evident when taking into account the major dopaminergic pathways in the mammalian brain: (a) the mesostriatal system consisting of dopaminergic neurons from the substantia nigra (SNc) innervating the striatum; (b) the mesolimbic system in which dopaminergic neurons from the ventral tegmental area (VTA) project into the hippocampus, nucleus accumbens (NAc) and amygdala; (c) the mesocortical system where DA neurons mostly from the VTA project into the cortical regions of the brain including the prefrontal cortex (PFx). Most of these regions have been implicated in depression. Furthermore, numerous studies support the hypothesis of decreased dopaminergic signalling in depression including reports that: (1) the severity of major depression correlates highly with patient response to amphetamine, a drug that facilitates increased synaptic DA levels through multiple mechanisms16, while another study has shown decreased levels of homovanillic acid, a major DA metabolite, in the CSF of depression patients17; (2) animals experiencing learned helplessness, a behavioural paradigm that recapitulates some of the symptoms of depression, have been shown to exhibit DA depletion in the striatum, which can be mitigated by pretreatments with DA agonists18-19; (3) motor effects induced by DA receptor agonists are increased after chronic treatment with antidepressants or electroconvulsive therapy20 suggesting reduced dopaminergic neurotransmission in depression; (4) in forced-swim tests, DA agonists have been shown to inhibit immobility, an indication of antidepressant activity, while DA receptor antagonists have been shown to inhibit the effects of antidepressants21-32; (5) DAT inhibitors nomifensine and bupropion have been shown to be effective antidepressants21, 33-34 and (6) clinical studies have also documented cases where DA receptor agonists have been effective in treating depression35-39. Furthermore, there is some evidence from neuroimaging studies that dopamine D2 receptor (D2R) are elevated in the striatum of depressed patients40-44.
Another brain pathway implicated in depression is the hypothalamic-pituatary-adrenal (HPA) axis. The HPA axis is involved in stress reaction and ultimately leads to increased secretion of glucocorticoids from the adrenal cortex. Although glucocorticoids have effects on the hippocampus it has also been shown to facilitate DA transmission in NAc45. In addition, frequent bouts of stress with intermittent exposure to glucocorticoids sensitize the mesolimbic DA system46. While the hippocampus and frontal cortex are undoubtedly involved in certain aspects of depression, symptoms of anhedonia, lack of motivation and motor deficits implicate other regions of the brain including the dorsal and ventral striatum, which are rich in dopaminergic neurons. Moreover, serotonergic activity has an impact on DA neurotransmission. Studies have shown that stimulation of 5HT1A receptors can stimulate DA release in PFx and NAc but inhibit DA release in the dorsal striatum47. Other studies have shown that activation of serotonergic raphe neurons reduces activity of dopaminergic neurons in VTA (not SNc) and inhibit locomotion, exploratory behaviour48-49.
Schizophrenia:
Schizophrenia is a severe chronic and debilitating mental disorder that strikes in youth and affects not only patients but their families and care-givers50-51. Approximately 2.2 million American adults have schizophrenia in a given year. Clinical symptoms of schizophrenia include delusions, hallucinations, disorganized thinking, and cognitive dysfunction that are divided into two major groups: positive and negative symptoms52.
Accumulated evidence suggests that the positive symptoms result from hyperdopaminergia involving dopamine D2 receptors in the limbic striatum, while the negative/cognitive symptoms arise from a hypodopaminergic function mediated by dopamine D1 receptors in the prefrontal cortex52. Despite decades of intensive research, current antipsychotic medications are still limited to the blockade of D2 receptor function that generally alleviate positive symptoms with only limited impact on cognitive and negative symptoms and can induce serious side effects including extrapyramidal side effects (EPS). Patients continue to experience significant disability and functional impairment that limits their integration in society. The unavailability of effective medications with both D1 agonism and D2 antagonism is mainly due to the unknown therapeutic target, a pathway through which both inhibition of D2 receptor and activation of D1 receptor function can be achieved.
The dopamine hypothesis of schizophrenia, in its original formulation addressed mainly positive symptoms53-54. Early pharmacotherapy for schizophrenia involved the use of reserpine, which blocks dopamine release from presynaptic terminals, and/or the use of antipsychotics55. Moreover, the most compelling evidence for the involvement of dopamine receptors in schizophrenia comes from the fact that most antipsychotics, including atypical antipsychotics, show a dose-dependent threshold of D2 receptor occupancy for their therapeutic effects55. The efficacy of both reserpine and antipsychotics in treating schizophrenia strongly implicate the involvement of dopamine in this neuropsychiatric disorder. More recent versions of this theory suggests that while the positive symptoms result from hyperdopaminergia in the limbic striatum, the negative/cognitive symptoms arise from a hypodopaminergic function in the prefrontal cortex (PFC)56-57. A significant body of literature lends support to this theory. PET & SPECT studies have shown evidence of increased dopamine synthesis, release, and levels in the subcortical/limbic regions58 while functional imaging studies have demonstrated hypofunction in the prefrontal cortex at baseline and while performing cognitive tasks59. More recent studies have focused on the dopamine D1 system in the PFC, as it is the predominant dopamine receptor sub-type in the PFC60-61, and show a decrease in receptor number which correlates with executive dysfunction62 and a compensatory up-regulation which correlates with working-memory dysfunction63. These clinical observations are well supported by preclinical evidence—PFC D1 receptor modulation changes the ‘memory fields’ of prefrontal neurons subserving working memory64-65 and D1 agonist administration improves working memory performance in both aging and dopamine deficient monkeys66 (as it does in aging human subjects)67-68.
Overview of DA Receptors:
In mammals, five distinct genes, termed D1/D5 for D1-like receptors and D2/D3/D4 for D2-like receptors, encode DA receptors. These receptors belong to a super-family of single polypeptide seven trans-membrane (TM) domain receptors that exert their biological effects via intracellular G-protein coupled signaling cascades1. D1 and D5 receptors preferentially couple to Gs proteins stimulating the activity of adenylate cyclase and PKA dependent pathways. D2 receptors display a more complex pattern of signal transduction primarily due to their coupling to subtype specific members of the Gi/Go protein family.
D2 receptors are known to stimulate a number of signal transduction pathways including the inhibition of adenylate cyclase activity, PI turnover, potentiation of arachidonic acid release, inwardly rectifying K+ and Ca2+ channels and mitogen activated protein kinases2. Moreover, studies have shown that protein interactions can play a large role in DA receptor function. For instance, the D2R has been shown to physically interact with Par-4. Interestingly, Par-4 mutant mice, which are unable to interact with D2R, exhibit depression-like behaviour3.
Dopamine D1-D2 Receptor Link:
While numerous studies have indicated a synergy between D1 and D2 receptors, an interesting study by Seeman et al (1989)69 provided the first evidence of a pharmacological link between D1 and D2 receptors. Briefly, it was shown that dopamine could lower the density of D2 receptors labeled by [3H] raclopride and that the addition of the specific D1 receptor antagonist, SCH-23390, prevented this reduction, suggesting a functional link between D1 and D2 receptors. Interestingly, this pharmacological D1-D2 link was absent or reduced in post-mortem brain tissues of approximately half of the schizophrenia population tested. However, it remains unclear if the absence of the D1-D2 link is due to an inherent dissociation between the D1 and D2 receptor that occurs in schizophrenia or is a result of antipsychotic drug treatment. Furthermore, several studies using a combination of D1 and D2 specific agonists and/or antagonists have shown that co-activation of D1-D2 receptors are required for long-term depression, anandamide-mediated memory consolidation and potentiation of immediate early gene response, suggesting a potential functional interaction between the D1R and D2R79-76.
Recent evidence indicates that D1 and D2 receptors form a protein complex, and co-activation of D1 and D2 receptors results in an increase of intracellular calcium levels via a signaling pathway not activated by either receptor alone, confirming the functional link observed between D1 and D2 receptors77-78. Furthermore, it has been shown that D1 and D2 receptors are co-expressed in neurons of the rat striatum, providing a basis for a functional interaction79-80.
Despite years of research in the field of mental health, there continues to be a need for new and improved medicines for treating psychiatric diseases and disorders, including depression, schizophrenia and psychotic symptoms thereof. The present inventors have accordingly sought to identify new diagnostic and chemotherapeutic methods in this area by investigating the functional association between D1 and D2 classes of DA receptors.